Thermodynamic and Kinetic Investigation on Electrogeneration of Hydroxyl Radicals for Water Purification

The water oxidation reaction (WOR) is of fundamental importance for the development of promising technologies in the field of energy conversion and environmental remediation. Although effort has been devoted to uncovering the mechanism of the oxygen evolution reaction (OER) by four-electron WOR, little attention has been paid to an in-depth understanding of one-electron WOR for •OH formation, which is the key to the electrochemical advanced oxidation process (EAOP) for environmental applications. Currently, oxygen evolution potential (OEP) is generally regarded as an important thermodynamic descriptor responsible for •OH production, but the kinetic descriptor has been overlooked; thus, the trade-off between thermodynamic and kinetic factors remains unclear. In this study, the thermodynamic feasibility and kinetic activity of electrogeneration of •OH were regulated by doping three kinds of low-electronegativity elements (Ce, Al, and In) into the prototype PbO2 nonactive anode. We investigated these electrodes in terms of •OH production and electrochemical decontamination performance in the context of sulfamethoxazole (SMX) removal. The results showed that 0.8%Ce-PbO2 anode (OEP = 3.67 V vs Ag/AgCl) achieved the highest SMX removal performance (∼100%; kobs = 0.027 min–1) and the highest •OH production with a relatively high number of •OH = 2.7 × 107. In comparison, the highest OEP (5.98 V vs Ag/AgCl) for the 1.6%Ce-PbO2 anode exhibited the lowest SMX removal (∼59%; kobs = 0.008 min–1) and 1 order of magnitude lower •OH production (relative number of •OH = 1.5 × 106). This suggested that the electrogeneration of •OH was unlikely to be always positively correlated to the OEP of the anode. Both experimental results and theoretical calculations verified the importance of suitably high OEP and relative fast electron transfer rate toward WOR to generate H2O+ (oxidation intermediate of H2O) for the ideal anode of EAOP. This study not only demonstrates the trade-off existing between thermodynamic (OEP) and kinetic (charge/electron transfer) factors responsible for electrogeneration of •OH via WOR fundamentally but also suggests a possible strategy for developing a high-efficiency anode by doping low-electronegativity elements to optimize one-electron WOR for environmental applications.